1. Field of the Invention
The invention relates to a mortise lock, and more particularly to a mortise lock with high resistance against destructive external pulling forces.
2. Description of the Related Art
A mortise lock typically includes a latch unit that includes a deadbolt and a live bolt and that can be operated through a key lock or an exterior handle at an outside of a door, or through a rotary button or an interior handle at an inside of the door. The key lock is formed with an external screw thread, and the latch unit has a threaded hole to receive the key lock and to engage the external screw thread. For installation in a door panel, the latch unit is disposed in a latch hole disposed within the door panel and opening at a side face of the door panel, and the key lock is inserted into the threaded hole of the latch unit through a lock hole formed in an outer face of the door panel. The exterior handle is attached to an exterior-cover and the exterior cover is attached to, an outside face of the door panel. The interior handle is attached to an interior cover, and the interior cover is disposed on an inside face of the door panel and is connected to the exterior cover. An example of such a mortise lock is disclosed in U.S. Pat. No. 7,152,442.
Generally, conventional mortise locks have no interlocking means to interlock the key lock with the exterior cover. Because retention of the key lock is mainly relied on an engagement between the screw thread of the key lock and the threaded hole in the latch unit, there is no sufficient resistance against external pulling forces, and the key lock can be easily removed from the door panel by burglars using destructive forces to violently pull the key lock and to destroy the screw thread thereof for separation of the key lock from the threaded hole of the latch unit.